This invention relates generally to gas turbine engines of the turbofan type and more specifically to a device for reducing stator source noise from the fan stage of such an engine.
In the recent efforts to reduce the noise emitted from modern high bypass ratio turbofan engines, considerable attention has been given to the fan sections. Two primary noise sources in these fan sections are known to be the fan rotor and the fan stators. In the effort to reduce the noise generated by these components, it has been necessary to first attempt to find the relative importance of the rotor and second, to attempt to understand the mechanisms by which the noise is generated.
Currently rotor and stator noise emissions are controlled principally through the use of acoustic linings in the engine nacelle, but these linings merely reduce the noise after it is generated rather than act upon the various sources of the noise. The only design technique presently employed to reduce the sources of rotor and stator noise is the choosing of the correct blade/vane ratio for minimizing blade passage tones at subsonic tip speeds.
Numerous contemporary investigations have been conducted to establish the relative importance of rotor and stator noise sources and to understand the various mechanisms by which the noise is generated. Improved experimental techniques have been developed which better simulate the conditions under which an engine operates in actual flight. Test data has indicated that noise from the rotor is dominating in the area in front of the engine, or the forward arc, whereas in the area after the engine, both rotor and stator noise are of equal significance. Recent tests have indicated, however, that stator noise is far more important in the forward arc than was previously believed. These tests have shown that when the fan is operating at subsonic relative tip speeds such as would occur during a landing approach, stator noise contributes significantly to the total noise in both the forward and aft arcs.
Stator noise is generally attributed to rotor-induced unsteady velocities interacting with the stator vanes. The major sources of these unsteady velocities are the wakes from the rotor blades and the secondary flows from the rotor tips and hub regions. Much effort has been expended to analyzing the effect of rotor wakes on the stators, but relatively little effort has been made in investigating the effect of rotor tip flows on them. A recent study has suggested that secondary flows originating from the rotor blade tips may be of greater significance than the remainder of the blade wakes.
Accordingly, it is an object of this invention to provide for an improved turbofan engine wherein a significant portion of the rotor tip flow bypasses the fan stators. It is also an object of this invention to provide for improved fan section for a turbofan engine having reduced stator source noise which can be adapted to present engines with minimal modifications. It is a further object of this invention to provide for an improved turbofan engine having reduced stator source noise during approach and takeoff conditions without affacting engine cruise performance. Another object of this invention is to provide for an improved turbofan engine in which a flexible membrane is disposed within the fan duct near the fan stators to vary the internal geometry of the duct.
It should be noted that the use of a flexible membrane to modify the internal geometry of certain sections of the fan duct of a turbofan engine has been suggested by others. For example, in U.S. Pat. No. 3,967,443, issued on July 6, 1976, to Davis Roberts McMurtry, the patentee discloses the use of a flexible flow area varying member attached to the centerbody near the fan duct exit for varying the nozzle area of the duct. He also suggests the use of a fluid pressure means for changing the shape of the member.
Also, in U.S. Pat. No. 3,611,724, issued on Oct. 12, 1971, to John P. Kutney, a sound suppression means for gas turbine power plant is described in which an inflatable diaphram is positioned within the inlet of the fan duct attached to either the fan casing or shroud. When the diaphram is inflated, the inlet area is reduced, choking the flow and, according to the claims of the inventor, substantially eliminating the propagation of forward arc noise from the fan rotor.